Certain molecular sieve catalysts, such as silicoaluminophosphate (SAPO) molecular sieves molecular sieve catalysts, are particularly susceptible to structural changes as a result of continued exposure to even low levels of moisture. Although such authorities as Paulitz et al., Microporous Materials, 2, 223-228 (1994), have shown through X-ray diffraction (XRD), nuclear magnetic resonance (NMR), infrared (IR) and nitrogen (N2) adsorption analyses that the structural change is largely reversible, such X-ray diffraction type studies have been found to be unreliable in determining loss of catalytic activity. For example, U.S. Pat. No. 6,316,683 (Janssen) indicates although such adsorption analyses show that structural change is largely reversible, that type of data cannot be relied upon as an indicator of loss of catalytic activity. According to Janssen, SAPO molecular sieve, as well as the SAPO molecular sieve blended with other catalyst material, can be protected from negative effects of moisture by properly shielding catalytic sites within the molecular sieve. Proper shielding can be accomplished in a variety of ways, including maintaining a template within the molecular sieve, covering the catalytic sites with a carbonaceous material or maintaining the sieve, even without a template, in an anhydrous environment.
U.S. Pat. Nos. 6,825,391 and 6,498,120 disclose another method of rejuvenating a SAPO molecular sieve. The method includes contacting a molecular sieve having a methanol uptake of less than 1, or a catalyst containing molecular sieve having a methanol up of less than 1, with anhydrous liquid or vapor until the methanol uptake ratio is increased by at least 10%. The rejuvenated molecular sieve or catalyst can be used to make an olefin product from an oxygenate-containing feedstock.
U.S. Pat. No. 6,537,941 also discloses a method of rejuvenating a SAPO molecular sieve. The method includes freeze drying a molecular sieve having a methanol conversion ratio of less than 1, or a catalyst containing molecular sieve and a binder having a methanol conversion ratio of less than 1. The rejuvenated molecular sieve or catalyst is used to make an olefin product from an oxygenate, and the olefin product can be separated into components that include ethylene and propylene, which can be used to make polyethylene and polypropylene, respectively.
U.S. Patent Publication No. 2005/0038306 details methods of starting up reaction systems that use metalloaluminophosphate molecular sieve catalysts that are susceptible to loss of catalytic activity due to contact with water molecules. The methods include loading activated catalyst into a reaction system and maintaining the catalyst at conditions so as to minimize or avoid loss of catalytic activity, until feed is fully introduced into the reaction system. In one embodiment, the reaction system is heated prior to introduction of the activated catalyst into the system. Alternatively, conditions of temperature of the catalyst, water partial pressure of the reaction system, and time of contact with water molecules are controlled so that loss of catalytic activity is not significant. These conditions are controlled so that the activated catalyst adsorbs little if any water. The greater the amount of water adsorbed, the greater the rate of deactivation of the catalyst.
The loss of molecular sieve catalytic activity as a result of contact with moisture presents a particular problem in the commercial production-to-use chain where storage and transport of molecular sieve and catalyst can take relatively long periods of time. For example, it is possible that molecular sieve or catalyst containing molecular sieve can be stored anywhere from 12 hours to many months, perhaps as long as one year before its use in a catalytic process. This stored sieve or catalyst is likely not to have a template within its internal pore structure as a result of having been removed by calcination prior to storage. Such a sieve or catalyst would be especially susceptible to damage by contact with moisture. Even partial loss of catalytic activity is of particular concern in large scale catalytic processes, which can include processes having a reactor loading in excess of 50 kg, particularly those in excess of 500 kg, and especially those in excess of 5000 kg. It would be particularly beneficial to find additional and more efficient ways in which to protect against loss of molecular sieve catalytic activity as a result of contact with moisture.